Fix isPlayable

src/GomokuCell.java

@@ -116,6 +116,7 @@ public class GomokuCell{
      * @return True if the current cell can be played with the condition of the gomoku.
      */
     public boolean isPlayable(){
+        if (!this.isEmpty()) return false;
         for (Cardinal c : Cardinal.values()) {
             GomokuCell current = this.getNeighbour(c);
             if (current != null && current.isPlayed()) return true;